1. Field of the Invention
Aspects of the present invention relate to a light scanning unit that scans an incident beam to be deflected onto a photosensitive medium and to an image forming apparatus using the same. More particularly, aspects of the present invention relate to a light scanning unit that has a structure for enhancing its sensitivity to curvature of a scanning line at a scanning surface due to a placement error of an f-θ lens and an image forming apparatus using the same.
2. Related Art
Generally, a light scanning unit is employed in image forming apparatuses such as laser beam printers, digital copiers, facsimile machines and multi-functional devices. Such a light scanning unit is used to deflect a laser beam irradiated from a light source and scan the laser beam in a main scanning direction of a photosensitive medium. An electrostatic latent image is formed on the exposed medium through scanning by the light scanning unit (referred to as “main scanning”) and by the movement of the exposed medium (referred to as “sub-scanning”).
The light scanning unit is also required to decrease the curvature of a scanning line on the scanning surface, which is associated with a placement error of an optical system.
In the light scanning unit, generation of the curvature of the scanning line due to the placement error of the optical system depends on a shape of an optical component and a sub-scanning magnification. The optical component usually has the shape of a lens in the main scanning direction, and its operation is influenced by whether light that has passed through a collimating lens is a parallel light or a converging light and by the number and location of f-θ lenses. In addition, an average and a deviation of the sub-scanning magnification are important factors in determining the magnification by the number and the location of the f-θ lenses and radii of curvature of an incident surface and an exit surface of the f-θ lenses.
Such a conventional light scanning unit is described in Japanese Laid-open Patent Publication No. 2004-184655 “OPTICAL SCANNER AND IMAGE FORMING APPARATUS USING SAME” (Jul. 2, 2004). For example, FIG. 1A and FIG. 1B show a conventional light scanning unit arranged to scan a laser beam in a main scanning direction of an exposed photosensitive medium. Referring to FIG. 1A and FIG. 1B, the conventional light scanning unit comprises a light source 1 that generates and irradiates a beam, a beam deflector 5 that deflects an incident beam to scan the beam irradiated by the light source 1 onto an exposed medium 8, and an f-θ lens 9 that corrects an error contained in the beam deflected by the beam deflector 5. The conventional light scanning unit may also include a diaphragm 2, a collimating lens 3 that collects the beam irradiated by the light source 1, via the diaphragm 2, to be a parallel beam, and a cylindrical lens 4 that shapes the beam. The diaphragm 2, collimating lens 3 and cylindrical lens 4 are installed in an optical path between the light source 1 and the beam deflector 5.
The beam deflector 5 comprises a rotary polygonal mirror rotated by a driving source (not shown). The beam irradiated by the light source 1 changes its direction while it is reflected by the rotating beam deflector 5, thus determining a scanning direction. That is, if the rotary polygonal mirror rotates in a direction A, the incident beam is reflected by a reflecting surface 5a of the rotary polygonal mirror and scanned in a main scanning direction B onto a scanning surface of the exposed medium 8.
The f-θ lens 9 comprises two lenses, a first lens 6 and a second lens 7. The first lens 6 has a meniscus shape in its main scanning sectional view, and thus has a positive refractive power. If “fm” is a focal distance in the main scanning surface of the exposed medium 8 and d1 is a center thickness of the first lens 6, a condition of d1/fm<0.06 is satisfied. The second lens 7 has a surface corresponding to the incident beam that is formed as a non-spherical surface or an oval surface without an inflection point, in a sub-scanning direction. If R is the radius of curvature on a beam axis in the main scanning surface sectional view of the surface corresponding to the incident beam, a condition of 2.5<|R/fm| is satisfied.
Therefore, according to the conventional light scanning unit, the curvature of the scanning line can be minimized by lens placement and conditions mentioned above when there is a parallel eccentricity that corresponds to a movement of the f-θ lens 9 in the sub-scanning direction.
On the other hand, since the conventional light scanning unit as described with reference to FIG. 1A and FIG. 1B is configured as an infinite optical system, where the beam that has passed through the collimating lens 3 is scanned parallel and two f-θ lenses 9 are used to improve the sensitivity to curvature of a scanning line, there is a disadvantage in that a degree of freedom of the lens placement is decreased or limited. Accordingly, if a tandem type light scanning unit is symmetrically configured to center around the beam deflector, the degree of freedom of an optical placement is inevitably decreased.
In addition, when two f-θ lenses 9 are used, even if the sensitivity is sufficiently decreased in an optical design, the possibility of occurrence of a placement error is high compared to when a single f-θ lens is used. As a result, the possibility of a lowered performance is increased. Moreover, the manufacturing cost is higher and likewise, the productivity in manufacturing the conventional light scanning unit is lower, compared to a light scanning unit having a single f-θ lens.